1. Field of the Invention
This invention relates to surgical correction of corneal astigmatism, myopia, and hyperopia and a corneal contact mask utilized to control the delivery of the light from an excimer laser.
2. Description of the Prior Art
Refractive surgery has been promoted in the United States and Russia over the past few years but its acceptance has been limited because of the poor predictability of the final optical results which include a resulting glare from incisions that encroach upon the optical zone. Techniques that rely upon the surgical production of corneal incisions have yielded inconsistent results because these surgical incisions in the cornea have been found to vary considerably in depth and length.
Laser keratectomy has been shown to be capable of yielding a more accurately controlled depth of corneal excision since each individual laser pulse excises a specific amount (0.2 to 1.0 mm) of corneal tissue. Accordingly, the depth of excised tissue is in theory uniform and predictable, provided that the energy distribution is homogeneous across the laser beam. Since the primary locus of astigmatism is in the cornea, surgical intervention for astigmatism is more important than for the correction of other refractive errors, especially since spectacle or contact lens correction is of limited value in compensating for large astigmatic errors.
The excimer laser was introduced to ophthalmology in 1983 (Trokel, S., et al, "Excimer surgery of the cornea," Am. J. Ophthalmol. 96: 710-715, 1983). The depth of incision with short intense pulses permitted great precision to be achieved in tests on freshly enucleated cow eyes. The photochemical laser-tissue interaction is not thermal, permitting direct breaks of organic molecular bonds without involving optical breakdown in adjacent tissue. Early experimental results in rabbits revealed problems of (1) corneal stromal swelling, probably in response to disturbed water relationships due to compromise of the epithelial barrier and severing of the lamellae and (2) rearrangement of endothelial cells resulting from loss of contact inhibition (Marshall, J., et al, "An ultrastructural study of corneal incisions induced by an excimer laser at 193 nm", Ophthalmology 92: 749-758, 1985). Experiments with freshly enucleated human eyes indicated that flattening obtained by excimer laser ablation correlated with results of clinical scalpel radial keratotomy, but evaluation of the effects on wound healing and possible damage to adjacent structures was not addressed (Cotliar, A. M., et al, "Excimer laser radial keratotomy,"Ophthalmology 92: 206-208, 1985). It was, however, suggested that this laser may become very useful in applications including penetrating and lamellar keratoplasty, keratomileusis, and epikeratophakia. Control of the area and depth of pulses using photolithographed masks resulted in ability to produce narrow cuts (20 .mu.m) and at depths depending on pulse number (Puliafito, C. A., et al, "Excimer laser ablation of the cornea and lens", Ophthalmology 92: 741-748, 1985). These controlled ablations had only very narrow bands of destruction at the adjacent edges. These studies led to the quantitation of laser ablation (Krueger, R. R. and S. L. Trokel, "Quantitation of corneal ablation by ultraviolet laser light", Arch. Ophthalmol. 103: 1741-1742, 1985). Excimer far UV radiation can be controlled to produce minimal adjacent tissue damage providing the angle and depth can be precisely controlled. The remaining problem of effects on healing could then be addressed.
Comparison of 193 nm and 248 nm ablation of precise disc sizes on plastic surfaces, and on rabbit and monkey corneas was undertaken by early workers in the field (Marshall, J., et al, "Photoablative reprofiling of the cornea using an excimer laser: Photorefractive keratectomy", Lasers in Ophthalmol. 1: 21-48, 1986). Properly controlled, healing was not impaired. It was evident, however, that irregularities in final surface contour were due to (1) diffraction of the rays, (2) surface impurities or debris in the target zone, and (3) movement of the target between laser pulses. Initial inhomogeneity in or on the surface results in a high resistance site to the ablation process and shields the underlying tissue.
Ablations applicable to myopic correction have been attained in human eye bank eyes of up to 7.5 mm in diameter using a rotating beam delivery system (Hanna, K. D., et al, "Excimer laser keratectomy for myopia with a rotating-slit delivery system", Arch. Ophthalmol. 106:245-250, 1988). Excimer laser ablation appears to be highly applicable to correction of astigmatism (Seiler, T., et al, "Excimer laser keratectomy for correction of astigmatism", Am. J. Ophthalmol. 105: 177-124, 1988). These investigators observed that corneal thickness increases peripherally and the laser beam is not perpendicular to the surface at the point of irradiance.
Wound healing was assessed in rabbits following excimer laser surface ablation (Hanna, K. D., et al, "Corneal stromal wound healing in rabbits after 193 nm excimer laser surface ablation", Arch. Ophthalmol. 107: 895-901, 1989). Healing appeared to be excellent except when over 85% to 90% of the corneal thickness had been cut. Endothelial cell disruption, junction separation and individual cell dropout occurred with corneal haze development with the deeper cuts. A delivery system designed to deliver predictable depths of cut is, therefore, essential. Similar findings were reported in studies on human blind eyes (Taylor, D. M., et al, "Human excimer laser lamellar Keratectomy", Ophthalmology 96: 654-664, 1989). Attention was directed to the challenges of improved procedures and equipment, the problems of individual variation, and the control of biologic responses to trauma before excimer laser lamellar keratectomy could become a clinically useful means of correcting refractive errors. In living monkey eyes, it was concluded that mild, typical wound healing occurred after excimer laser keratomileusis (Fantes, F. E., et al, "Wound healing after excimer laser keratomileusis [photorefractive keratectomy] in monkeys", Arch. Ophthalmol. 108: 665-675, 1990). All corneas were epithelialized by 7 days. By 6 weeks, mild to moderate haze was apparent with clearing by 6 to 9 months. The epithelium was thickened at 21 days after ablation, but returned to normal by 3 months. Subepithelial fibroblasts were three times the density of normal keratocytes, but returned to nearly normal numbers by 9 months. One conclusion reached was that control of the contour and uniformity of the ablated surface is important for structural and biological responses of the cornea.
It has been observed that less haze and surface irregularities occur with tangential keratectomies of rabbit corneas in comparison to en face methods (Holme, R. J., et al, "A comparison of en face and tangential wide-area excimer surface ablation in the rabbit").
Another concern is the fluorescence spectra associated with ablation of corneal tissue. Wavelengths greater than 400 nm pass to the retina and may be phototoxic to the lens and to the retina. Studies designed to assess these concerns at ablative levels by 193 nm excimer lasers indicate that the fluorescence spectra can be attributed to the irradiated site rather than to luminous ablation products or laser-produced plasma. Thus, the phototoxic risk from the quantum yield of fluorescence from corneal ablation appears to be very slight (Tuft, S., et al, "Characterization of the fluorescence spectra produced by excimer laser irradiation of the cornea", Invest. Ophthalmol. Visual Sci. 31: 1512-1518, 1990).
Review of the literature clearly reveals that far UV vaporization (ablation with an excimer laser at 193 nm, for example) is a feasible means to sculpture or reprofile the cornea to correct nearsightedness, farsightedness, astigmatism, corneal scars, corneal densities, etc. The healing appears to parallel or to be equal to healing after scalpel intervention, providing the proper guidelines for pulsing and duration are followed. There remains a need to control the contour and uniformity of the ablated surface. Such control will reduce the adverse structural and biological response of the cornea and insure that a desired corrective change results.
The use of a mask of nearly identical optical density to the cornea that can be preformed on the surface of cornea so as to provide a smooth surface of exact contour would solve many of the problems still remaining which thus far have prevented precise control of laser beam keratotomy. Such a mask would be required to withstand exposure to moist gases directed tangentially to the corneal surface throughout the duration of exposure to the laser to remove ablated debris. The modulation of the beam energy distribution of the laser in a controlled fashion should also be provided by such a corneal mask. The use of a smooth ablatable mask having a known contour and having the density of the cornea would aid in insuring accurate direction and depth of a tangential cut utilizing a laser beam. The ablatable mask of the invention provides such advantages.
In "Pluronic Polyol, a Potential Alloplastic Keratorefractive Material", Journal of Cataract Refractive Surgery, Vol 14, May, 1988, Kim et al disclose the use of a polyol sold under the trademark PLURONIC.RTM. which is described as prepared by condensating propylene oxide on a propylene glycol nucleus followed by the condensating ethylene oxide onto both ends of the poly(oxypropylene) base. These polymers were evaluated as a potential material for alloplastic keratorefractive surgery in which the material in a liquid state was injected into a surgically prepared axial 7 mm mid-stromal corneal bed in rabbits. Post-operative follow-up over a period of three months indicated that the material was well tolerated by the cornea and provided refractive flattening of the cornea so as to obtain about 3 diopters change.
In U.S. Pat. No. 4,188,373, PLURONIC polyols are disclosed as forming aqueous compositions which are liquids when cool and gel upon heating. Adjusting the concentration of the polymer provides the desired sol-gel transition temperature, that is, the lower the concentration of polymer, the higher the sol-gel transition temperature, after crossing a critical concentration minimum, below which a gel will not form. Other polyoxyalkylene gel compositions are disclosed in U.S. Pat. Nos. 4,810,503 and 4,879,109.
Ionic polysaccharides have been used in the application of drugs by controlled release. Such ionic polysaccharides as chitosan or sodium alginate are disclosed as useful in providing spherical agglomerates of water-insoluble drugs in the Journal of Pharmaceutical Sciences, volume 78, number 11, November 1989, Bodmeier et al. Alginates have also been used as a depot substance in active immunization, as disclosed in the Journal of Pathology and Bacteriology, volume 77, (1959), C. R. Amies. Calcium alginate gel formulations have also found use as a matrix material for the controlled release of herbicides, as disclosed in the Journal of Controlled Release, 3 (1986) pages 229-233, Pfister et al. Alginates have also been used to form hydrogel foam wound dressings, as disclosed in U.S. Pat. No. 4,948,575.